1. Field of the Invention
This invention relates to amorphous metallic alloys, and more particularly to amorphous alloys consisting essentially of iron, boron, silicon, and carbon which find uses in the production of magnetic cores used in the manufacture of electric distribution and power transformers.
2. Description of the Prior Art
Amorphous metallic alloys (metallic glasses) are metastable materials lacking any long range atomic order. They are characterized by x-ray diffraction patterns consisting of diffuse (broad) intensity maxima, quantitatively similar to the diffraction patterns observed for liquids or inorganic oxide glasses. However, upon heating to a sufficiently high temperature, they begin to crystallize with the evolution of the heat of crystallization. Correspondingly, the x-ray diffraction pattern begins to change to that observed from crystalline materials, i.e., sharp intensity maxima begin to evolve in the pattern. The metastable state of these alloys offers significant advantages over the crystalline forms of the same alloys, particularly with respect to the mechanical and magnetic properties of the alloy.
For example, there are commercially available metallic glasses which have only about a third of the total core losses of those of conventional crystalline 3 wt. % Si-Fe grain-oriented steels, in applications as magnetic cores of electrical distribution transformers. (See, for example "Metallic Glasses in Distribution Transformer Applications: An Update", by V. R. V. Ramanan, J. Mater. Eng., 13, (1991) pp. 119-127). Considering that there are about 30 million distribution transformers in the U.S. alone, which consume about 5 billion pounds of magnetic core material, the potential for energy savings and the associated economic benefits resulting from the use of metallic glasses in distribution transformer cores can be substantial.
Amorphous metallic alloys are produced generally by rapidly cooling a melt using any of a variety of techniques conventional in the art. The term "rapid cooling" usually refers to cooling rates of at least about 10.sup.4 .degree. C./s; in the case of most Fe-rich alloys, generally higher cooling rates (10.sup.5 to 10.sup.6 .degree. C./s) are necessary to suppress the formation of crystalline phases, and to quench the alloy into the metastable amorphous state. Examples of the techniques available for fabricating amorphous metallic alloys include sputter or spray depositing onto a (usually chilled) substrate, jet casting, planar flow casting, etc. Typically, the particular composition is selected, powders or granules of the requisite elements (or of materials that decompose to form the elements, such as ferroboron, ferrosilicon, etc.) in the desired proportions are then melted and homogenized and the molten alloy is then rapidly quenched at a rate appropriate, for the chosen composition, to the formation of the amorphous state.
The most preferred process for fabricating continuous metallic glass strip is the process known as planar flow casting, set forth in U.S. Pat. No. 4,142,571 to Narasimhan, assigned to Allied-Signal Inc. The planar flow casting process comprises the steps of:
(a) moving the surface of a chill body in a longitudinal direction at a predetermined velocity of from about 100 to about 2000 meters per minute past the orifice of a nozzle defined by a pair of generally parallel lips delimiting a slotted opening located proximate to the surface of the chill body such that the gap between the lips and the surface changes from about 0.03 to about 1 millimeter, the orifice being arranged generally perpendicular to the direction of movement of the chill body, and
(b) forcing a stream of molten alloy through the orifice of the nozzle into contact with the surface of the moving chill body to permit the alloy to solidify thereon to form a continuous strip. Preferably, the nozzle slot has a width of from about 0.3 to 1 millimeter, the first lip has a width at least equal to the width of the slot and the second lip has a width of from about 1.5 to 3 times the width of the slot. Metallic strip produced in accordance with the Narasimhan process can have widths ranging from 7 millimeters, or less, to 150 to 200 mm, or more. The planar flow casting process described in U.S. Pat. No. 4,142,571 is capable of producing amorphous metallic strip ranging from less than 0.025 millimeters in thickness to about 0.14 millimeters or more, depending on the composition, melting point, solidification and crystallization characteristics of the alloy employed.
Understanding which alloys can be produced economically and in large quantities in the amorphous form and the properties of alloys in the amorphous form has been the subject of considerable research over the past 20 years. The most well-known disclosure directed to the issue--What alloys can be more easily produced in the amorphous form?--is U.S. Pat. Re No. 32,925 to H. S. Chen and D. E. Polk, assigned to Allied-Signal Inc. Disclosed therein is a class of amorphous metallic alloys having the formula M.sub.a Y.sub.b Z.sub.c, where M is a metal consisting essentially of a metal selected from the group of iron, nickel, cobalt, chromium, and vanadium, Y is at least one element selected from the group of phosphorus, boron and carbon, Z is at least one element form the group consisting of aluminum, antimony, beryllium, germanium, indium, tin and silicon, "a" ranges from about 60 to 90 atom %, "b" ranges from about 10 to 30 atom % and "c" ranges from about 0.1 to 15 atom percent. Today, the vast majority of commercially available amorphous metallic alloys are within the scope of the above-recited formula.
With continuing research and development in the area of amorphous metallic alloys, it has become apparent that certain alloys and alloy systems possess magnetic and physical properties which enhance their utility in certain applications of worldwide importance, particularly in electrical applications as core materials for distribution and power transformers, generators and electric motors.
Early research and development in the area of amorphous metallic alloys identified a binary alloy, Fe.sub.80 B.sub.20, as a candidate alloy for use in the manufacture of magnetic cores employed in transformers, particularly distribution transformers, and generators because the alloy exhibited a high saturation magnetization value (about 178 emu/g). It is known, however, that Fe.sub.80 B.sub.20 is difficult to cast into amorphous form. Moreover, it tends to be thermally unstable because of a low crystallization temperature and is difficult to produce in ductile strip form. Further, it has been determined that its core loss and exciting power requirements are only minimally acceptable. Thus, alloys of improved castability and stability, and improved magnetic properties, had to be developed to enable the practical use of amorphous metallic alloys in the manufacture of magnetic cores, especially magnetic cores for distribution transformers.
Subsequent to additional research, ternary alloys of Fe-B-Si were identified as superior to Fe.sub.80 B.sub.20 for use in such applications. A wide range of alloy classes, with their own unique set of magnetic properties, have been disclosed over the years. U.S. Pat. Nos. 4,217,135 and 4,300,950 to Luborsky et al. disclose a class of alloys represented generally by the formula Fe.sub.80-84 B.sub.12-19 Si.sub.1-8 subject to the provisos that the alloy must exhibit a saturation magnetization value of at least about 174 emu/g (a value presently recognized as the preferred value) at 30.degree. C., a coercivity less than about 0.03 Oe and a crystallization temperature of at least about 320.degree. C. Freilich et al. in U.S. patent application Ser. No. 220,602, filed Sep. 23, 1981, assigned to Allied-Signal Inc., disclosed that a class of Fe-B-Si alloys represented by the formula Fe.sub..about.75-78.5 B.sub..about.11.about.21 Si.sub..about.4.about.10.5 exhibited high crystallization temperature combined with low core loss and low exciting power requirements at conditions approximating the ordinary transformer operating conditions of magnetic cores in distribution transformers (i.e. 60 Hz, 1.4 T at 100.degree. C.), while maintaining acceptably high saturation magnetization values. U.S. patent application Ser. No. 235,064 discloses that a class of alloys defined by the formula Fe.sub.77-80 B.sub.12-16 Si.sub.4-10 exhibit low core loss and low coercivity at room temperature after aging, and have high saturation magnetization values. In U.S. Pat. No. 5,035,755, assigned to Allied-Signal Inc., Nathasingh et al. disclose a class of alloys useful for manufacture of magnetic cores for distribution transformers, which are represented by the formula Fe.sub.79.4-79.8 B.sub.12- Si.sub.6-8, and which alloys exhibit unexpectedly low core loss and exciting power requirements both before and after aging, in combination with an acceptably high saturation magnetization value. Finally, U.S. patent application Ser. No. 479,489 filed Feb. 13, 1990 to Ramanan et al., assigned to Allied-Signal Inc., disclosed yet another class of Fe-B-Si alloys with high iron contents exhibiting improved utility and handleability in the production of magnetic cores used in the manufacture of electric distribution and power transformers. It is disclosed that these alloys have the combination of high crystallization temperature, high saturation induction, low core loss and low exciting power requirements at 60 Hz and 1.4 T at 25.degree. C. over a range of annealing conditions, and improved retention of ductility subsequent to anneals over a range of annealing conditions.
In other research efforts to redress the deficient characteristics in Fe.sub.80 B.sub.20, and to recover some of the saturation magnetization "lost" from the Fe-B system, the ternary Fe-B-C alloys were taught to have great promise. The properties of alloys in this system are summarized in a comprehensive report by nuborsky et al. in "The Fe-B-C Ternary Amorphous Alloys", General Electric Co. Technical Information Series Report No. 79CRD169, August 1979. It is disclosed in this report that while a high saturation magnetization value persists over a wider range of compositions in the Fe-B-C system when compared with the Fe-B-Si system, the beneficial effects found from Si (in Fe-B-Si alloys) on increased crystallization temperatures, and, therefore, alloy stability, were seriously compromised over much of the composition region in the Fe-B-C alloys. In other words, crystallization temperatures usually were reduced when C replaced B. From a magnetic property perspective, a major drawback noted from the Fe-B-C alloys was that the coercivities of these alloys were higher than those of the Fe-B-Si alloys and higher, even, than that of the binary Fe-B alloy. Primarily as a result of these deficiencies in alloy stability and coercivity, the Fe-B-C alloys have not been pursued further, since the time of the Luborsky et al. report, as possible commercially significant alloys for application in magnetic cores of transformers for electrical distribution.
A class of amorphous metallic Fe-B-Si-C alloys represented by the formula Fe.sub.80-82 B.sub.12.5-14.5 Si.sub.2.5-5.0 C.sub.1.5-2.5 are disclosed by DeCristofaro et al. in U.S. Pat. No. 4,219,355, assigned to Allied-Signal Inc. which alloys are disclosed to exhibit, in combination, high magnetization, low core loss and low volt-ampere demand (at 60 Hz), and wherein the improved ac and dc magnetic characteristics remain stable at temperatures up to 150.degree. C. DeCristofaro et al. also disclose that Fe-B-Si-C alloy compositions outside of the above formula possess unacceptable dc characteristics (coercivity, B.sub.80 (induction at 1 Oe), etc.), or ac characteristics (core loss and/or exciting power), or both.
Amorphous metallic Fe-B-Si-C alloys are also disclosed in U.S. Pat. No. 4,437,907 to Sato et al. In this patent, it is taught that there is a class of alloys described by the formula Fe.sub.74-80 B.sub.6-13 Si.sub.8-19 C.sub.0-3.5, which alloys exhibit a low core loss at 50 Hz and 1.26 T and high thermal stability of magnetic properties, and in which alloys, there is, after aging at 200.degree. C., a high degree of retention of magnetic flux density measured at 1 Oe at room temperature and a good degree of retention of core loss at the above mentioned conditions.
It is readily apparent from the above discussion that researchers focused on different properties as being critical to the determination of which alloys would be best suited for the manufacture of magnetic cores for distribution and power transformers, but none recognized the combination of properties necessary for clearly superior results in all aspects of the production and operation of magnetic cores and, consequently, a variety of different alloys were discovered, each focusing on only part of the total combination. More specifically, conspicuously absent from the above recited disclosures is an appreciation for a class of alloys wherein the alloys exhibit a high crystallization temperature and a high saturation magnetization value, in combination with low core loss and low exciting power requirements after having been annealed over a wide range of annealing temperatures and times and, in addition, retain sufficient ductility over a range of annealing conditions to ease magnetic core production. Alloys which exhibit this combination of features would find overwhelming acceptance in the transformer manufacturing industry because they would possess the magnetic characteristics essential to improved operation of the transformer and more readily accommodate variations in the equipment, processes and handling techniques employed by different transformer core manufacturers.
The element boron in the amorphous metallic alloys discussed above is the major cost component in the total raw material costs associated with these alloys. For example, in the case of the Fe-B-Si alloys discussed above, 3 percent by weight (about 13 at. %) of boron in an alloy could represent as much as about 70% of the total raw material costs. In addition to the desirable combination of features described above for a transformer core alloy, if such an alloy could have lower boron levels in its composition, thereby allowing reduced total production costs in large scale manufacture of the alloy for transformer applications, a more rapid implementation of amorphous metallic alloy cores would occur, with the attendant societal benefits discussed previously.